Remove OracleDB tests

[ricardograca]

• Related Issues: Node 7 CI tests fail due to oracledb install failure. #1713, Reduce complexity for dev testing setup & update CONTRIBUTING.md #1661, Establish sensible maintainer team structure #1660, Running test with npm won't work because oracledb #1605
• Previous PRs: OracleDB tests will be loaded and executed only when oracledb is installed #1609, Add oracledb integration tests. #1528

Introduction

Remove all OracleDB related tests and associated code.

Motivation

The tests for OracleDB are an endless cause of problems and build errors that have nothing to do with Bookshelf. To make matters worse, the entire test suite must run on the slower sudo: required Travis infrastructure which usually entails very long wait times before the build even starts (it can take hours), as well as taking double the time to run the test suite itself due to the extra OracleDB installation steps.

This change is also in line with the ideas expressed in #1600 and #1661 of removing all database adapter tests and leave that concern to the query builder being used (Knex). Since Knex already tests against several databases, including OracleDB, there shouldn't be any reason to duplicate those tests in Bookshelf.

Proposed solution

This change simply removes all OracleDB tests, which shouldn't be a problem anyway, since we don't have any database specific code. Since removing tests for all other databases is going to require more work, it's best to start with the database that is easier to remove and that is causing more problems.

Removing OracleDB tests already made the TravisCI build much faster which is valuable in terms of development and quality of life for the developers.

Current PR Issues

Doesn't remove all database adapter tests.

Alternatives considered

Finding a maintainer for OracleDB support within Bookshelf, but that would be short lived anyway, since the plan is to move away from tests with actual databases and instead test the behavior of Bookshelf alone.

Separating OracleDB tests into a separate build matrix that is allowed to fail, but that would require a little bit of work to adapt the test suite, and again is against the objectives already mentioned.